1. Field of the Invention
The present invention relates to an image encoding apparatus and, more particularly, to a binary image hierarchical encoding apparatus in a still image communication apparatus.
2. Related Background Art
A facsimile apparatus as a typical still image communication apparatus adopts a system for sequentially scanning, encoding and transmitting an image. This system requires a long transmission time since all image data must be transmitted, and is difficult to be apply to image communication services such as an image data base service, videotex, and the like, which require quick judgment of an image. In order to realize such a service, a system different from that employed in the facsimile apparatus is proposed ("Sequential Reproduction/Encoding System of Facsimile Signal Suitable for Conversational Image Communication", Endo and Yamasaki, Shingakuron (B), J67-B12, pp. 1462-1469 (1984)). In this system, when one image is transmitted, rough image information is transmitted first, and additional information is transmitted later, to generate detailed image data.
However, since the rough image information to be transmitted first is generated by omitting pixels at specific intervals from an original image, valid image information often cannot be transmitted at an initial stage, depending on the type of image. For example, a straight line having a width of one pixel may be deleted.
When dynamic arithmetic encoding is employed in the sequential reproduction/encoding system, if images at respective stages are classified to the same number of states, the number of symbols assigned to each state is decreased at a stage having a low resolution. For this reason, encoding is ended before a skew value in dynamic arithmetic encoding can be sufficiently converged, resulting in poor encoding efficiency.